Metallocene catalysts can be rendered heterogeneous by impregnation of a porous solid support with organometallic entities. Thus, EP 2 067 94 teaches the simultaneous or subsequent impregnation of a silica by an MAO and a metallocene. These compounds are then adsorbed on the support. However, the active entities are observed to easily desorb from the support during their use. Catalysts of this type then lead to problems of fouling of the polymerization reactors. In addition, a nonhomogeneous polymerization result is observed. To overcome this disadvantage, one solution consists in chemically grafting the organometallic entity to the support.
Several routes are possible in obtaining this type of catalyst.
One route consists of the unit construction on an inorganic support: the catalyst is constructed stage by stage on a support. Grafting to the support is carried out either via cene ligands or via a substituent of the bridge of the metallocene complex.
Thus, Soga et al. describe (Macromol. Symp., 1995, 89, 249–258) a process comprising a heat treatment and then the chemical modification of the silica to introduce an intermediate group. The attachment of the cene ligands is subsequently carried out via their lithiated equivalents. These catalysts are unfortunately not very active. Furthermore, the nature of the active sites obtained is not well defined. It would appear that two types of entities grafted to the silica coexist: a monobridged entity and a bibridged entity, rigidly bonded to the support.
EP 0 821 009 provides for the construction of a metallocene on a support treated with butyllithium by the intermediate composed of an ethyl cyclopentadienyl as anchoring chain.
Patent U.S. Pat. No. 5,846,895, discloses the synthesis of catalysts having a ligand grafted to the support via a substituent of the ligand, the second ligand being “free” with regard to the surface. The cyclopentadienyl (Cp) ligand is always bonded to the silane before being grafted to the support. However, the activity of the catalyst remains far below that of the equivalent homogeneous catalyst.
A second route consists in grafting a presynthesized catalyst: a metallocene catalyst, already synthesized, is reacted with an inorganic support via a substituent present on one of its cene ligands. The grafting is carried out in a single stage.
This route has a number of advantages, including that of good definition of the structure of the metallocene. The support is generally treated chemically beforehand in order to increase its reactivity with respect to the substituent of the cene ligand, which will make it possible to increase the efficiency of the grafting. Also, although this process comprises only one stage, it remains necessary to carry out numerous stages of synthesis and of purification of the presynthesized catalyst. Finally, there is a risk of obtaining several different entities, resulting from the reaction of different sites of the presynthesized catalyst with the support. Patents U.S. Pat. Nos. 5,202,398 and 5,824,620 disclose a supported catalyst obtained by direct reaction of a functionalized metallocene with the support. A catalytic system obtained by heterogenization of a metallocene by an —OSiR3 group on a support is also disclosed in EP 0 839 836. However, this approach exhibits the disadvantage of requiring the synthesis beforehand of the functionalized metallocene and of reducing the versatility of the synthesis of catalysts for the various requirements of the polyolefin industry.
Finally, the unit construction on an organic support constitutes a third route for producing solid catalytic components: the synthesis of the catalyst is carried out either on a polymer which has already been synthesized, for example polystyrene, or on a polysiloxane which comprises cene ligands.
However, and whatever the methods employed, catalysts of low activity are generally obtained.